Gas source molecular beam epitaxial growth and device applications in In0.5Ga0.5P and In0.5Al0.5P heterostructures

158

Thhl Solid Fihns, 231 (1993) 158-172

Gas source molecular beam epitaxial growth and device applications in Ino.sGao.sP and Ino.sAlo.sP heterostructures J. M . K u o A T & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 (USA)

Abstract This article reviews briefly the recent progress in the In0.hGao.hP and In0.hAlo.hP epilayers lattice-matched to GaAs grown by gas source molecular beam epitaxy (GSMBE). It covers the growth conditions, and the structural, electrical, and optical properties. Important research results on the electronic and photonic device applications of these materials grown by GSMBE are also reviewed and discussed. We focv.5 on the lnGaP/InGaAs and lnAIP/InGaAs pseudomorphic modulation-doped field-effect transistors, InGaP/Ga ~and InAIP/GaAs heterojunction bipolar transistors, short wavelength visible laser diodes, and 0.98 p.m lasers u Lg Ino.hGao.hP as the cladding layers. The results clearly demonstrate that high quality ln0.hGao.hP and In0.hAlo.~P heterostructures prepared by GSMBE are suitable for state-of-the-art device applications.

I. Introduction

independence between the lattice constant and the energy band gap: the lattice constant depends strongly on the InP mole fraction of the quaternary, while the energy band gap depends primarily on the AlP mole fraction. Unlike other quaternary compounds, compositional grading can be achieved by changing only two group III fluxes. Recently, considerable effort has been devoted to the growth of this material system and the realization of visible laser diodes as well as visible light-emitting diodes (LEDs) emitting in the

During the past several years much progress has been made in the growth as well as device fabrication of I n G a P and related compound semiconductors Ino.5(Al.,.Gat_.,-)o.sP lattice matched to GaAs. Ino.5(Al_,.Ga~ _x)o.sP has a wide direct energy band gap between 1.9 and 2.3eV and can be used as active and confinement layers in heterostructure optoelectronic devices. As shown in Fig. 1, it features a relative

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Elsevier Sequoia

J. M. Kuo / GSMBE-grown Ino.sGaosP and lnosdlo.sP heterostructures

580-680 nm regime, heterojunction bipolar transistors (HBTs) and modulation-doped field effect transistors (MODFETs). Although Ino.5(A1,.Ga~_x)o.sP films have been prepared by elemental source molecular beam epitaxy (MBE) [1, 2], metalorganic MBE (MOMBE) [3] and gas source MBE (GSMBE) [4], by far most of them are grown by metalorganic chemical vapor deposition (MOCVD) [5, 6]. For device applications, the preparation of abrupt heterostructures of high quality is important and therefore the precise control of epitayer thickness and composition with atomic accuracy is inevitably required for the crystal growth technique employed. Although MBE has excellent ability in controlling epilayer thickness and composition, little work has been done on the growth of phosphorus-containing compound semiconductors owing to the operational difficulties associated with the handling and control of MBE phosphorus sources and also to the limited material quality achievable, which is mainly limited by the purity of red phosphorus charges available [7]. Recently, the use of a valved, solid cracker source for the generation of phosphorus beams in MBE has provided an attractive alternative to the use of phosphine and may open the door for MBE to grow phosphorus-containing materials [8]. GSMBE offers a number of advantages over other growth techniques. The epitaxial process requires a much lower flow of hydrides than MOCVD. As opposed to MOMBE, the growth kinetics and alloy composition of ternary and quaternary materials are almost independent of substrate temperature. The absence of metalorganic compounds in the growth chamber avoids any carbon contamination of dopant effusion cells, cracking cells as well as hot parts of the growth chamber. Significant progress has been made by GSMBE in the growth of InGaP and related materials [9-15]. Here some of the recent important research results achieved by GSMBE are presented and discussed. The first part of this article is predominantly concerned with the growth and properties of Ino.sGao.sP and Alo.5Ino.sP. Their structural, electrical and optical properties are reviewed. The second part of the article is concentrated on the electronic and photonic device applications of these materials grown by GSMBE. Emphasis is on MODFETs, HBTs, short wavelength visible laser diodes and 0.98 ~tm lasers using Ino.sGao.sP as the cladding layers.

2. GSMBE growth and properties of lno.s(AIx Ga ~_x)o.sP epilayers 2.1. Gas source molecular beam epitaxy GSMBE is similar to the MBE process where only the molecular beams of group III and group V elements

159

are undergoing interaction during the course of the epitaxial growth [16]. In GSMBE growth, 100% AsH3 and PH 3 gases are used as the group V beam sources, while group III sources of Ga, AI and In are supplied as conventional solid sources [17]. The n- and p-type dopants are Si and Be respectively. A gas-handling system with mass flow controllers and switching valves is used to introduce pure AsH 3 and PH3 into the growth chamber through a low pressure cracking cell. The operating temperature of the cracker is typically 950-1100°C, where the P2:P4 ratio is maximum and the cracking efficiency is as high as possible. Most GSMBE systems are equipped with a 2200 1 s-~ turbomolecular pump to provide adequate pumping of hydrogen produced by the decomposition of the hydrides. Additional cryopumps or diffusion pumps can be added to increase the pumping speed. Depending on preference and the material systems, one shared cracker can be used for uniformity considerations or two independent crackers can be chosen to minimize the interdiffusion problem at the interface when switching AsH3 and PH 3 gases.

2.2. Growth of lno.sGao.sP and Ino.sAlo.sP In0.sGao.sP and Ino.sAlo.sP grown by GSMBE on GaAs have been reported by several groups [9-15]. The pumping configuration may be different and the growth conditions may not be the same. Substrate heat cleaning is done under an As2 flux produced by the thermal decomposition of AsH3 in a low pressure cracker held at high temperature. A (2 x 4) As-stabilized reflection high energy electron diffraction (RHEED) pattern is observed after the desorption of the oxide. A GaAs buffer layer is necessary in order to subsequently grow mirror-like InGaP and InA1P epilayers. Growth interruption is necessary to set the InGaP and InA1P growth temperatures, because they are usually grown at 400580 °C, lower than the GaAs growth temperature. Normally, the RHEED pattern changes to a (2 x 3) reconstructed surface at the end of interruption. Switching in the Pz flux before the growth of undoped InGaP and InAIP will usually change the RHEED pattern to a (1 x 2) pattern. During the growth of InGaP and InAIP a (2 x 1) RHEED pattern is observed. The PH 3 flow rate and InGaP/InAIP growth rate are typically 2.5 sscm (standard cm 3 min -t) and 0.9 lam h -t respectively. It should be noted that long exposure of the GaAs surface to P2 flow causes surface roughening and spotty RHEED patterns. This surface reaction is considered to be due to a large binding energy of GaP in comparison with that of GaAs. The surface roughness of GaAs exposed to the P beam can presumably be explained by the formation of islands to reduce misfit strain of the GaPAs layer [15].

160

J. M. Kuo / GSMBE-grown Ino sGao.sP and Ino sAlosP heterostructures

2.3. Structural properties of Ino.sGao.sP and Ino.sAlo.sP R H E E D oscillations of InP on InP, G a A s on GaAs and AlAs on GaAs are used to predetermine the cell temperatures of In, G a and A1 for lattice-matched I n G a P and InAIP grown on GaAs. We use double-crystal X-ray ( D C X R ) diffraction in the (400) configuration to determine the lattice mismatch and film composition by applying Vegard's law. Attaining a lattice match Aa/ a < 5 x 10-4 is relatively easy and reproducible with these techniques. The full width at half-maximum ( F W H M ) for I n G a P 1.8 gm thick grown in our system was measured to be only 19 arcsec. Similar results were also reported by Masselink et al. [ 11]. These are the narrowest ever reported for I n G a P grown by any technique. Transmission electron diffraction ( T E D ) patterns of both [il0] and [110] are taken for the study of ordering. In [110] zone axis T E D patterns, only the reflections due to the zinc blende structure are observed, while extra wavy diffuse streaks in the ( 1, 1, n)/2 equivalent positions with a continuous value of n are observed at the [100] zone axis for I n G a P grown at about 500 °C. At lower growth temperatures, no wavy streaks or extra diffraction spots are observed in the [110] T E D patterns. When the extra spots or diffuse streaks are present, the diffraction patterns correspond to a CuPttype crystalline structure and are similar to those observed for I n G a P grown by M O C V D [18, 19]. It is believed that the T E D patterns reveal wavy streaks instead of perfectly round (111)/2 spots because of ordered G a I n G a I n - I n G a I n G a antiphase boundaries in the (111) plane [19]. The degree of ordering is found to be less perfect than that for M O C V D owing to the lower growth temperature [15]. 2. 4. Electrical properties of lno.s Gao.5P and Ino.5A lo.5P Undoped I n G a P is always found to be n type, with a background concentration in the low 10 j5 cm -3 range. Hall mobilities of nominally undoped I n G a P are typically around 1800-2500cm2V -I s -I. The best results obtained in our laboratory [10] are / ~ = 3 2 0 0 c m 2 V - I S - 1 and n = 1.3 x 10 ]5 cm -3, which are comparable with the best reported values for MBE [1] and M O C V D [20]. At 77 K the corresponding mobility and carrier concentration were 25 700 cm 2 V - t s - ~ and l x 10~5cm -3 respectively. The electrical activity is found to be unity up to an atomic Be concentration of 2 x 1018 cm -3 in InA1P and 3 x 1019 cm -3 in InGaP, the electrical activity being defined as the ratio of the hole concentration to the atomic Be concentration. The hole concentration tends to saturate about 3 x 1018 c m - 3 in InA1P and 4 x 1 0 t9 c m - 3 in InGaP. A further increase in Be-doping level results in a marked decrease in hole concentration. The m a x i m u m hole concentrations achieved are 3.5 x 10 t8 c m - 3 in InA1P and 4 x 1019 c m - 3 in I n G a P [15].

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2.5. Optical properties of Ino.sGao.sP and Ino sAlo.sP The optical quality of the the I n G a P and InA1P epilayers can be investigated by cathodoluminescence (CL). We have used CL to find the optimum growth conditions for I n G a P [10]. Under optimum growth conditions a typical CL spectrum at room temperature is as shown in Fig. 2. A single peak is observed and the F W H M is about 29 meV, which is the narrowest ever reported. The CL emission wavelength is about 652 nm. After correcting for the lattice mismatch, a room temperature band gap of about 1.92 eV is obtained. It is reported by Quigley et al. that the low temperature photoluminescence (PL) exhibits a single peak when I n G a P is grown under optimum conditions [9]. At 13 K the peak position is at 1.944 eV for x = 0.484, which is due to a donor-to-valence band transition. On the other hand, samples grown under non-optimum conditions exhibit a second and much weaker peak at about 38 meV below the primary peak, which is believed to be due to a d o n o r - a c c e p t o r transition. Low temperature PL with an F W H M of 6.7 meV is reported by Biswas et al. [12], which compares with the best value for I n G a P grown by M O C V D [21]. In general, our understanding of PL spectra is well behind that of G a A s and considerable uncertainty still persists concerning the identification of the peaks. More work has to be done in the future.

3. Ino.sGao.sP/Ino.sGao.sAs M O D F E T s The absence of deep traps in doped I n G a P makes I n G a P an excellent alternative to AIGaAs in M O D FETs. A1GaAs contains D X centers which cause large threshold voltage shifts and drain I - V collapse at low

J. M. Kuo / GSMBE-grown Ino.sGao.sP and Ino.sAlo.sP heterostructures

temperatures. Previously, InGaP/GaAs M O D F E T s have been grown by MOCVD and demonstrated a negligible deep trap effect at low temperature [22]. Here we review the results for InGaP/InGaAs M O D F E T s grown by G S M B E in our laboratory [23]. A pseudomorphic InGaAs rather than the lattice-matched GaAs channel was used to further enhance device performance. The device structures consist of an undoped GaAs buffer 5000 ,~ thick followed by a 150 ~ undoped InGaAs strained channel and a 30/~ undoped InGaP spacer. Si-doped InGaP (n = 2 × 1018 cm -3) 200 thick is used as a donor layer and a 150 ~ undoped InGaP layer is employed to improve the Schottky contact. Finally, a 200/~ n+-GaAs cap layer is added to improve the ohmic contacts.

3. I. Results of Hall measurements After removing the top n+-GaAs cap layer, a Hall mobility of 4460 cm 2 V -t S - 1 with n~ =2.1 x 10 t2 cm -2 at 3 0 0 K was measured, while the mobility was 15 700 c m 2 V - I s - l with n, = 1.64 x l012 c m - 2 at 77 K. The sheet charge density increased by only 4% at 77 K after illumination. This light insensitivity of the sheet charge density indicates that trapping effects in this material system are insignificant at low temperature.

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3.2. Fabrication of InGaP/InGaAs MODFETs Devices were processed by a conventional optical lithography technique. An N H 4 O H - H 2 0 2 - H 2 0 solution was used for etching GaAs and Ino.2Gao.sAs layers. This etchant proved to be selective for InGaP and only attacked GaAs. It therefore provided a convenient way to deposit the gate metal on the undoped I n G a P without any risk of uncontrolled gate recess variation such as in the case of etching the d o n o r layer. The etching of InGaP for the mesas was achieved by using an HC1-H3PO 4 solution. Ohmic contacts were realized by using Ge/Au/Ni/Ti/Au (700/1400/500/200/I000/~) followed by a 400 °C, 1 min hot plate annealing. After etching away the top n+-GaAs layer, gates 1 ~tm long (Ti/Au, 500/3000 ]k or WSi,.) were deposited by the lift-off process. Finally, Ti/Au was used for the interconnection level. Figure 3 shows the fabricated MODF E T structures. 3.3. Device character&tics Figure 4 shows the d.c. characteristics of a 1 ~tm× 150~tm gate InGaP/InGaAs M O D F E T at 300 K. Excellent channel pinch-off and an extremely low output conductance gds of 2 m S m m -t were achieved. A peak extrinsic gm of 205 mS m m - ~ at 300 K was measured (Vds = 2.5 V, Vgs = -- 1.6 V). The gm value observed in this pseudomorphic M O D F E T shows a 25% improvement compared with that reported previ-

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ously for a lattice-matched InGaP/GaAs M O D F E T with the same gate dimensions [22]. The d.c. gain ratio gm/gds is over 100, which is essential for high speed and microwave circuit applications. An extremely low reverse leakage current of 28 ~A m m - I was measured at Vgs = - 5 V, indicating the high quality of the Schottky contact. Figure 5 shows the d.c. characteristics of the same device at 77 K. The extrinsic gm was enhanced to 270 mS mm -t, while good channel pinch-off and a low g~s were maintained. The threshold voltage shift between 300 and 77 K was only 75 mV, indicative of insignificant trapping at cryogenic temperatures. One of the most significant properties of InGaP is the absence of donor-related deep traps. In conventional A1GaAs/GaAs M O D F E T s these so-called DX centers

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band discontinuity AEc increases linearly with increasing A1 composition from x = 0 to 0.7 and reaches a maximum value of 0.38 eV at x = 0.7 [24]. For x > 0.7 the band structure becomes indirect and AE~ decreases slightly toward 0.31 eV with increasing AI composition. The Ino.sAlosP/Ino.2GaosAs pseudomorphic modulation-doped structure not only has a higher AEc but also preserves the transport properties of the InGaAs channel; therefore it is of great interest to investigate InA1P MODFETs. In this section we shall report our recent results with InAlP/InGaAs M O D F E T s [25, 26].

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will trap carriers in the conducting channel and cause current collapse and a V,h shift at low temperatures. We conducted bias stress measurements at low temperatures in the dark to confirm this advantage. Under high channel bias operations, electrons launched from the source terminal could be scattered into donor layers near the drain owing to the high field real space transfer mechanism. The existence of deep traps in the donor layers will trap the scattered electrons at low temperature and result in a decrease in carriers in the channel. Current collapse together with a V,h shift could therefore be observed in this case. After applying a 10 k V c m -~ electric field, no current collapse was observed in our InGaP/InGaAs pseudomorphic MODFETs and the threshold voltage was shifted by only 56 mV. This result confirms our previous discussion that trapping effects are negligible in this material system under cryogenic temperature operation. Microwave characteristics measured using an HP8510B network analyzer together with a Cascade probe station showed a unity-current-gain cut-off frequency fv of 12.2 GHz and an fmax of 26.5 G H z at V d s = 4 V and Vgs= - 1 . 9 V .

4. lno.sAlo.sP/Ino.sGao.sAs M O D F E T s 4.1. Motivations A figure of merit for M O D F E T s is the high currentdriving capability, which is related to the heterojunction band discontinuity. In principle, the larger the band discontinuity is, the higher is the sheet carrier density and therefore the higher is the current-driving capability. Ino.5(Alx Gal_ x)0.sP/GaAs has the largest band gap difference AEg among all I I I - V semiconductor heterojunctions lattice matched to GaAs. The conduction

4.2. Device structures The device cross-section is shown in Fig. 6. An undoped GaAs buffer 5000 A thick was grown followed by a 150 ,~ undoped In0.2Ga0.s As pseudomorphic channel and 20 ~ GaAs and 30 ,~ InAIP undoped spacers. A 200,~ Si-doped InA1P layer (n = 2 x 10~Scm -3) provided the donors and a 150 ~ undoped InA1P layer was used to improve the Schottky contact. Finally, a 200 n+-GaAs cap was grown to reduce the contact resistance. 4.3. Results o f Hall measurements The Hall mobility was 5390 and 27 300 cm 2 V ~sat 300 and 77 K respectively, while the corresponding electron concentrations were 2.24 x 10 ~2 and 1.55 × 10 ~2 cm -2. After removing the n+-GaAs cap layer, the mobilities increased to 6880 and 29 700 cm 2 V -~ s-~ at 300 and 77 K respectively, while the corresponding sheet carrier densities became 1.32 x 10 ~2 and 1.3 x 10 ~-"cm--'. The sheet charge density increased by only 5% under illumination at 77 K, indicating that deep trap effects are not significant in this material system.

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J. M. Kuo / GSMBE-grown Ino. 5 Gau. 5 P and lno~,41~j.5P heterostructures

4.4. Device characteristics Figure 7 shows the room temperature I - V characteristic of a device with a 1 gm x 75 lam gate. The device showed excellent pinch-off and a very low output conductance o f only 1.3 mS m m -t. The transconductance and transfer characteristics are shown in Fig. 8. The maximum extrinsic transconductance was 173 mS m m - t at 300 K with V~ = 2 V. An extremely low gate reverse leakage current was achieved ( I ~ < 2 5 0 n A at Vg~ = - 5 V). The gate breakdown voltage was - 17 V. A 0.92 eV Schottky barrier height was evaluated from C - V measurements. The turn-on voltage was 0.7 V and the ideality factor was 1.27, indicating that thermionic emission and diffusion are the dominant mechanisms of the current flow. At 77 K the device showed a maximum transconductance o f 2 8 3 m S m m -~ in the dark. The threshold voltage shift was only 30 m V from the value at 300 K. This threshold shift is much smaller than that of

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Alo.15Ga0.85As/InxGal_,As (0.05 ~< x ~<0.2) M O D FETs [27], indicating that the DX-like trap density in InAIP is not high. The same stress bias experiments applied to the InGaP/InGaAs M O D F E T s were applied to the InA1P/InGaAs M O D F E T s . Figure 9 shows the 77 K I - V characteristics o f the device before and after bias stress in the dark. Figure 10 shows the corresponding transconductances and transfer curves. N o I - V collapse was observed and the threshold voltage shift was only 18 mV. This again confirms that there is not an inordinate number o f DX-like centers in the InA1P grown by GSMBE. 4.5. Microwave character&tics Figure 11 shows the short-circuit current m a x i m u m available gain as a function o f for the device. A maximum oscillation fm,x = 26 G H z and a current gain cut-off

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J. M. Kuo / GSMBE-grown Ino.sGaosP and Ino.sAlosP heterostructures

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fT = 1 1.5 G H z have been obtained for devices of I gm gate length.

4.6. High temperature characteristics Typical I - V characteristics of the WSix-gated MODFETs at 200 and 300 °C are shown in Figs. 12 and 13 respectively. At 200 °C the device still showed very good channel pinch-off together with an extremely low output conductance of 2.2 mS mm -~. A peak extrinsic transconductance of 79 mS m m - ~was revealed. At 300 °C the device showed degradation. The variation in gr, and gas with temperature is shown in Fig. 14. The rates of change in gd, and V,h increase very rapidly above 250 °C, indicating that AEc at the InA1P-InGaAs heterointerface is no longer adequate to confine the hot electrons in the drain region.

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Fig. 14. Temperature dependence of the transconductance and output conductance of an InAIP/InGaAs MODFET.

4. 7. MODFET summary It is clear that the properties of In0.sGa0.sP and In0.sAlo.sP grown by GSMBE are suitable for MODF E T applications. DX-like deep trap effects are insignificant in both materials. The low threshold voltage shifts at cryogenic temperatures and the successful operation of these M O D F E T s up to 250 °C suggest that they have potential for cryogenic and high temperature applications.

5. Heterojunction bipolar transistors 5.1. Ino.sGao.sP/GaAs HBTs 5.1.1. Introductory remarks Mondry and Kroemer reported the first N - p - n heterojunction bipolar transistor using an InGaP emitter on GaAs substrates [28]. InGaP/GaAs offers many potential advantages over AIGaAs/GaAs for use in

J. M. Kuo / GSMBE-grown InosGao.sP and Ino.sAlo.sP heterostructures

HBTs owing to its favorable band alignment. A m o n g these are improved carrier injection efficiency at the b a s e - e m i t t e r junction due to its smaller conduction band discontinuity as well as larger valence band discontinuity [24, 29-34] and better etching selectivity relative to G a A s [35]. Although a higher current gain is expected, early results were not encouraging because of material problems. As epitaxial growth techniques improved, the advantages of I n G a P / G a A s HBTs were revealed. Previously most of the work concentrated on M O C V D - g r o w n materials [36-41], though recently InG a P / G a A s HBTs have been grown by M O M B E [42, 43] and G S M B E [34]. In this section the realization of I n G a P / G a A s HBTs by G S M B E in our laboratory will be reported. The advantages of G S M B E will be accented. 5.1.2. H B T structures The structures of the devices studied in our laboratory are shown in Fig. 15. They were grown by G S M B E on semi-insulating (100) G a A s substrates. The base was doped with Be to 8 x 1018 or 2 x 1019 c m -3 and was separated from the n - I n G a P emitter by a 150 or 200 ~ undoped G a A s spacer layer. On top of the spacer was a 5 0 0 A I n G a P emitter Si doped to 5 X 1017cm -3 followed by a 300/~ n + - I n G a P layer doped to 1 x 1019 cm -3. A 2000/~ n+-GaAs layer and a 1000 A grading contact layer of n + - I n G a A s completed the emitter cap layers. The collector region consisted of a 3000 A G a A s layer Si doped to 2 × 1016 cm -3 on top of a 3000 ~ n+-GaAs subcollector layer.

165

5.1.3. Device characteristics Large area HBTs were fabricated using circular mesa structures. The e m i t t e r - b a s e and collector-base junction areas were 6.4 × 10 -5 and 4.3 x 10 -4 cm 2 respectively. Figure 16 shows the collector and base currents of an H B T with a base doping of 2 × 10 j9 cm -3 as a function of e m i t t e r - b a s e voltage. The measured ideality factors were 1.03 for I c and 1.42 for I b. When the base doping was reduced to 8 × 1018 cm -3, the ideality factors changed to 1.05 and 1.51 for I c and I b respectively. These are in good agreement with the calculated values, indicating that the GSMBE-grown I n G a P / G a A s HBTs have abrupt e m i t t e r - b a s e junctions. Figure 17 shows the c o m m o n emitter I - V characteristics of a typical I n G a P / G a A s H B T with 2 × 1019 cm -3 base doping. The offset voltage is only about 120 mV. The device exhibits a d.c. current gain of 122 and a small current gain of 128 at a current density of 1600 A cm -2. For HBTs with 8 × 1018 cm -3 base doping, d.c. and small signal current gains as high as 250

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Fig. 16. Oummel plot for an N-p-n InOaP/OaAs HBT with base doping of 2 x 1019cm-3.

I O0 -

IB

501.tA/step

80

S.I. GaAs SUBSTRATE

Fig. 15. Epitaxial layer structure of the GSMBE-grown InGaP/GaAs N - p - n HBT. The space layer thickness is 200.~ for a 2 x 10t9cm-3 base-doped HBT.

0

0

1

2 VCE( V )

3

4

Fig. 17. Common emitter I - V characteristics of a typical InGaP/ GaAs HBT with base doping of 2 × 10j9 cm -3. The emitter is 90 lam in diameter.

J. M. Kuo / GSMBE-grown hlo sGao sP and lno sAlo,sP heterostructures

166

300 _

VCE = 2.0V

300

Z ,,~ 240 (5 I..18o rr" I~

240 Z .< (.9 I.18o nrr

12o

12o

0'<

0a 60 0 10-6

up is still worthy of high current gain N - p - n HBT fabrication. Furthermore, the high selectivity of both wet and dry etching between InAIP and GaAs makes the fabrication of InAIP/GaAs HBTs very controllable [45]. It is therefore of interest to explore HBTs with an InA1P emitter. We shall discuss our recent results for InAIP/GaAs HBTs.

60

10-5

10-4

10-3

10-2

0 10-1

Ic(A) Fig. 18. Typical d.c. and a.c. current gains o f an 8 x 10tScm -3 base-doped H B T as a function o f collector current m e a s u r e d at a collector voltage of 2 V.

and 320 respectively have been achieved. Figure 18 shows typical d.c. and a.c. current gains as a function of collector current at a collector voltage of 2 V. The current gains are high for the high base dopings. We attribute this to the high quality of InGaP and the abruptness of the I n G a P - G a A s heterointerface grown by GSMBE. It is noteworthy that HBTs employed with the emitter-edge-thinning technique at the emitter-base junction can reduce the surface recombination currents and further increase the current gain [34]. 5.2. Ino sAlo.sP/GaAs H B T s 5.2.1. Motivations The motivations for investigating InAIP/GaAs N p - n HBTs are as follows. It has been reported that the valence band discontinuity AEv of the In05(A1, Ga~ _.00.sP/GaAs heterojunction increases linearly with increasing A1 composition and reaches its maximum value at the I n A I P - G a A s interface (0.62 eV) [24]. This heterojunction also produces the largest AEv among all I I I - V semiconductor heterojunctions lattice matched to GaAs. Therefore, in terms of AEv, it is the most desirable band line-up for N - p - n HBTs. Compared with In0.sGa05P/GaAs HBTs ( A E v = 0 . 3 0 e V ) , the larger AEv can more effectively reduce the back injection of holes from the base into the emitter and therefore a higher current gain is expected. On the other hand, the indirect band structure [44] of the InAIP results in a smaller conduction band discontinuity (AE~ = 0.31 eV) than for the direct band structure case [24]. Although AE~ of InAIP/GaAs is larger than that of InGaP/GaAs, which may lower the amount of electrons transferred, the larger conduction band discontinuity of the emitter junction may result in high energy electron injection into the base. Overall, the band line-

5.2.2. Device characteristics The InAIP/GaAs HBT structures consist of a GaAs buffer layer (300 nm, n = 4 x 1018 cm-3), a GaAs collector layer (450nm), a GaAs base layer (100nm, p = 1 x I019cm-3), a GaAs spacer layer (15nm), an InA1P emitter layer (250nm, n = 5 x 1017cm-3), an InA1P capping layer (15 nm, 4 x 10~8cm-3), a GaAs capping layer (n = 4 x 10 ~8cm -3) and an InGaAs capping_!ayer (5 nm, 1 x 1019 c m - 3 ) . An InAIP stop etch layer of 10 nm is inserted between the GaAs subcollector and collector. Large area devices with an emitter area of 1001am x 1501am are fabricated using the wet chemical etching process without employing the emitter-edge-thinning technique. The common emitter current-voltage ( I - V ) characteristics of a typical InAIP/GaAs HBT are shown in Fig. 19. A current gain as high as 300 and an offset voltage as small as 80 mV were measured. This current gain is higher than those obtained from InGaP/GaAs HBTs. We attributed this to the larger AEv of the InAIP/GaAs heterojunction. Note that the offset voltage was small even though no grading was used in the emitter-base junction. These results indicate that InAIP/GaAs is indeed a good material system for HBT structures. The Gummel plot is shown in Fig. 20. The collector current has an ideality factor of 1.02 over six orders of magnitude. This suggests that the diffusion and thermionic emission currents dominate the collector current up to 1.05 V. The ideality factor calculated for an abrupt p - n heterojunction agrees well with the measured value, indicating that the HBT has an abrupt emitter-base junction and there is no significant Be out-diffusion into the InA1P emitter. Further evidence of good emitter-base junction quality is shown by the useful current gain even in the microampere regime. This is in contrast with the A1GaAs/GaAs HBTs. The base current exhibits an ideality factor of 1.62, which is comparable with those of the A1GaAs/GaAs and InGaP/GaAs HBTs. 5.3. H B T summary In summary, InGaP/GaAs and InA1P/GaAs HBTs have been fabricated and characterized. Experimental results demonstrate the advantages of the band alignment of these material systems, making these HBTs attractive candidates for high speed digital circuit applications.

J. M. Kuo / GSMBE-grown hlo ~Gao.5 P and hTo 5 Alo ~ P heterostructures

167

Fig. 19. Common emitter 1 - V characteristics of a typical lnAIP/GaAs HBT. The base doping is 1 x 10 ~9 cm -3.

10 0

~

i

10 0

I

InAeP/GaAs HBT

/

10"2

10-2

#

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10 -8

0

0.6

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t

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i

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2.4

r

3.0

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[nA1P/GaAs HBT.

6. Visible lasers

Short wavelength semiconductor lasers are of great interest for use in optical information and recording systems such as high density optical disks, high speed

laser printers and bar code readers. The most promising material system for laser emission in the 580-680 nm range is InAIGaP. Since the first successful continuous wave (CW) operation of an-InA1GaP double-heterostructure (DH) lasei" was achieved in 1985 [46-49], InA1GaP visible lasers have been studied extensively and remarkable progress has been made in the device characteristics. To date, there have been many reports on the fabrication and performance of InA1GaP visible lasers, most having been prepared by MOCVD. One of the most serious difficulties with MOCVD is the high resistivity of the p-type cladding layer, which tends to increase with increasing aluminum content of In(A1Ga)P quaternary alloys. The high resistivity is mainly attributed to low hole concentrations. Heavily doped p-type cladding layers improve the characteristics of visible laser diodes [49]. With continuing efforts to shorten the wavelength toward yellow and green emission, GSMBE plays a more important role. GSMBE has the advantage of providing heavy p-type doping in InGaP and InA1P [14]. This is an essential and superior property necessary to realize low threshold current operation, especially in the short wavelength region, in comparison with MOCVD. In this section we will only cover the key factors in improving the performance of visible laser diodes by GSMBE.

168

J. M, Kuo / G S M B E - g r o w n In~ 5 Gao.5 P and In~ 5 AI o 5 P heterostructures

6. I. Superlattice confinement layers The first room temperature CW operation of an InGaP/InAIP visible laser grown by G S M B E was achieved by inserting short period InGaP/InAIP superlattice confinement (SLC) layers between the InGaP active layer and the InAIP cladding layers [50]. A drastic reduction in the threshold current density from 3.5 to 1.6 k A c m -2 was observed. The PL intensity from the InGaP active layer with the SLC layer was 20 times greater than that from the InGaP clad only with an InA1P layer. This is attributed to the reduction in the heterointerface recombination velocity and thus the lengthening of the carrier lifetime. A maximum light output of 10.5 mW at 669 nm was obtained under CW operation at room temperature for a ridge stripe laser with a stripe width of 6 ~m. 6.2. Misoriented substrates Substrate misorientation was first used for lasers grown by MOCVD in order to suppress the effects of spontaneous crystal ordering [51 - 53]. Systematic investigation of the substrate misorientation effects in GSMBE showed a remarkable reduction in the threshold current density with increasing substrate misorientation angle from the (100) surface toward the [011] direction [54]. For stripe lasers 1.I mm long and 55 ~m wide with a 60 nm InGaP bulk crystal active layer grown on a 15° misorientation angle (100) substrate, a minimum threshold current density of 702 A cm -2 was measured. The utilization of misoriented substrates is very effective in obtaining high performance visible lasers. The misorientation effects may be induced by improving the heterointerface quality, enhancing the electrical activity of p-type doping in the InAIP cladding layers or suppressing the spontaneous crystal ordering of the InGaP active and InA1P layers. 6.3. Strained quantum well active layers One major step in the realization of very low threshold current density and high output power of visible laser diodes was the introduction of strained quantum well active layers [55, 56]. In comparison with a conventional index-guided lattice-matched single-quantum-well (SQW) visible laser, a 25% increase in the differential gain coefficient and a 60% decrease in the transparency current density have been found experimentally by incorporating a 0.65% compressively strained single-quantum-well (SSQW) active layer in the MOCVD-grown structures. GSMBE-grown SSQW InGaP/InA1P visible lasers have been reported to have a threshold current density of 329 A cm -2 at a wavelength of 700 nm [57]. The SSQW visible lasers consisted of an InGaP strained quantum well active layer sandwiched between InGaP/ InA1P SLC layers and were grown on 15° off-cut (100) Si-doped GaAs substrates toward the [110] direction.

6.4. Multiquantum barriers Since most of the energy band gap difference of InGaP/InA1P lies in the valence band, rapid increases in threshold current densities occur at high temperatures or under high current injection conditions of the visible lasers. These problems arise from the leakage of carriers over the conduction heterobarrier. Because of the small effective mass of electrons, a larger AEc is needed to confine the electrons more effectively. The limitation of the heterobarrier height imposed by the inherent material properties can be overcome by the proposed multiquantum barriers (MQBs) [58] in which electron wave interference can enhance the heterobarrier height. The enhanced carrier confinement effect of MQBs in 660 nm InGaP/InA1P multiple-quantum-well (MQW) lasers with SLCs was demonstrated through the observation of the high temperature characteristics of the threshold [59]. The MQBs consisted first of a 10 nm InA1P barrier and then InGaP/InA1P MQWs with constant or modulated InGaP well widths. Five quantum wells of InGaP (16.7,~)/InAIP (30,~) were prepared for constantthickness MQBs, while the thicknesses of the modulated InGaP wells were varied sequentially as 7.1, 12, 16.7, 21.4 and 26.2 A,. Theoretical calculation of both MQBs predicted that the heterobarrier can be enhanced by at least 100mV over the bulk potential barrier between InGaP and InAIP. Threshold current densities as low as 840 A cm -2 and a characteristic temperature To as high as 167 K were reported [59].

7. 0.98 ~m lasers

Recently, the development of 0.98 I~m semiconductor lasers as pumping sources for erbium-doped fiber amplifiers has progressed rapidly. The traditional approach using AIGaAs for the cladding layers in these lasers may have a long-term reliability problem due to facet oxidation, which results in laser degradation. By replacing A1GaAs with Ino.sGao.sP for the cladding layers, less degradation due to the oxidation of aluminum during the fabrication process and laser operation is expected, since the laser is aluminum free [60, 61]. In this section we review the first Ino.2Gao.sAs/ GaAs/Ino.sGao.sP strained layer quantum well ridge waveguide lasers grown by GSMBE and demonstrate the regrowth capability of GSMBE on the patterned InGaP layer to make the first self-aligned index-guided laser.

7. I. Ridge waveguide lasers A separate confinement heterostructure (SCH) was used to provide confinement of electrical carriers and the optical field. The device structure is shown in Fig. 21. The epitaxial layers were grown by GSMBE in one

169

J. M. Kuo / G S M B E - g r o w n Ino. 5 Gao. 5 P and Ino.5 AIo5 P heterostructures

40

°dl

3 35 AuBe/Ti/Au p+-GaAs CAP p-fnGaP CLADDING Si3N4 p-GaAs ETCH STOP i-InGaAs/GaAs SCH QWs

l }

~

95

CW 115

30

c~ 2 5 W

O 2O I..--

n-InGaP CLADDING n+-GaAs SUBSTRATE

O t0 5 0

°C

Fig. 21. Schematic diagram of the self-aligned ridge waveguide InGaAs/GaAs quantum well laser with [nGaP cladding layers.

growth step. The active region consisted of three 70 Ino.2Gao.8As strained quantum wells and two 200~, GaAs barriers. The active region was sandwiched between two 800 ~ GaAs waveguide layers. The p-doped and n-doped InGaP cladding layers were 1.5 lam thick and were lattice matched to GaAs within 5 x 10 -4. A thin GaAs stop etch layer was placed in the upper InGaP cladding layer to control the etch depth of the ridge waveguide lasers. An n+-GaAs and 10-period GaAs/InGaP buffer layers were grown before the n-InGaP cladding layer to improve the crystal quality. Broad area lasers thus grown have a threshold current density of 177 A cm -2 for a cavity 1016 lam long, a low internal waveguide loss of 9.1 cm -~ and a high internal quantum efficiency of 91% [61]. Ridge waveguide lasers [62] of width 3 lam have very low threshold currents: 7 mA for cavities 254 gm long and 12 mA for cavities 508 lam long at room temperature under CW operation conditions, as shown in Fig. 22. An external differential quantum efficiency as high as 0.9 mW mA- t was obtained for lasers 254 lam long. A very low internal waveguide loss of 7.7 cm-' and a very high internal 3 >(3 Z w 2 (3 ,T

100 150 200 CURRENT (mA)

300

quantum efficiency of 92.5% have been deduced. Figure 23 shows the CW L - I characteristics of an antireflection-high-reflection-coated (AR-HR-coated) 3 lam x 508 ~tm laser at temperatures from 30 to 185 °C. The reflectivities of the AR- and HR-coated facets were approximately 5% and 90% respectively. The highest CW operating temperature of 185 °C is comparable with the best performance reported (200 °C) for the 980 nm laser with Alo.6Gao.4As cladding layers grown by MOCVD [63], which has a larger heterobarrier for electron confinement. The characteristic temperature To is 180 K between 30 and 60 °C. The high power performance is shown in Fig. 24. A peak power.as high as 160 mW is obtained before reaching catastrophic optical damage. Fundamental mode operation is obtained up to 90 mW with 0± = 48 ° and 011= 13°. 200 RT, CW 3 ~tm x 500 lxm AR/HR /

/

RT, CW 40 E

250

Fig. 23. CW light vs. current characteristics of an A R - H R - c o a t e d InGaAs/GaAs/InGaP ridge waveguide quantum well laser (3 ~tm x 508 lain) at temperatures from 30 to 185 °C.

A 160

50 cm -1 rl i = 92.5% ct i = 7.7

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-80 -40 0 40 80 FAR-FIELD ANGLES(deg)

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I 200

I I I 400 600 800 CAVITY LENGTH (~tm)

100

I 1000

1200

Fig. 22. Inverse external differential quantum efficiency and threshold current v s . cavity length of the InGaAs/GaAs/lnGaP ridge waveguide lasers.

i

I

i

200 CURRENT (mA)

I

300

r

400

Fig. 24. CW light vs. current curve of a 3 lam x 508 lam A R - H R coated InGaAs/GaAs/InGaP ridge waveguide laser at room temperature. The peak output power is 160 roW. The inset shows the far-field pattern of the laser at 80 roW.

d. M. Kuo / GSMBE-grown lno~Ga, sP and InosAlosP heterostructures

170

7.2. Index-guided lasers We have successfully demonstrated the first indexguided self-aligned Ino.zGao.sAs/GaAs/Ino.5 Gao.5P 0.98 pm quantum well lasers grown by GSMBE in two growth sequences [64]. The InGaP surface can desorb the native oxide more easily than AIGaAs. A schematic diagram of the device structure is shown in Fig. 25. The first growth sequence consisted of 10 pairs of n+-doped InGaP/GaAs superlattices as the buffer layer, a 1.5 jam n+-InGaP cladding layer, a 1000 ~ GaAs SCH layer, three Ino.2Gao.sAs/GaAs (70/ 200 A) strained quantum wells as the active medium, a 1000 A GaAs SCH layer, a 1000 ]k p+-InGaP barrier layer, a 100/~ p+-GaAs stop etch layer and a 2000 ]k n+-Ino.sGao.sP blocking layer. After the first growth the wafer was patterned with SiO2 window stripes of various widths. The stripes were aligned along the (110) crystal orientation to provide etching profiles with positive slope. Channels of widths 2.5, 4.5, 7.5 and 12 jam were delineated by removing the exposed n+-InGaP blocking layer by selective wet chemical etching. After removing the SiO2 etch mask and the exposed thin p+-GaAs stop etch layer, the wafer was reloaded into the GSMBE system for the second growth sequence. The all-InGaP surface is important for regrowth, because the exposure of the GaAs surface in the phosphorus flux at high temperature during the heat clean process causes an interdiffusion problem. The second growth consisted of a 1000,~ p+-GaAs waveguide layer, a 1.0 lam p+-InGaP cladding layer and a 2000/~ p+-GaAs cap layer. Previously, most self-aligned laser structures utilized a patterned low band gap (high refractive index) GaAs layer for current blocking and guiding of the lateral mode [65]. The lateral mode in these structures is stabilized by the loss as well as the refractive index difference from the embedded antiguiding small band gap confining layer. In our structures the patterned wide band gap n+-InGaP layer is reverse biased as the

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.:~,~,~:~,~,~,-~.....~.,,~....~...,,,..:..~,,,,..+,,: / - p-InGaP CLADDING :..'~L?..:.~.tq.:~:x L.:-"x'x':':'~-'~: :': / p-GaAs WAVEGUIDE ,,~','. ~ ~'~*,'.%'.~.,','.'.~:,:.:.:,:,?,:,'.'.',',',',',','..,... ~':~:!:i:i:i:~/ /.. n-InGaP BLOCKING n-InGaP CLADDING } i-fnGaAs/GaAs SCH QWs n-InGaP

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30

0

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o 1

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F

0

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200

I

I

I

400 600 800 1000 CAVITY LENGTH (p.m)

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Fig. 26. Cavity length dependence of the CW threshold current and reciprocal external differential efficiency of uncoated self-aligned lasers 2.5 p.m wide.

current confinement layer. To guide the lateral mode, the regrown high index p+-GaAs waveguide layer, which fills in the opening of the low index n+-InGaP blocking layer, couples the optical field from the active medium to constitute a large optical cavity and provides the necessary index guiding. Figure 26 shows the cavity length dependence of the CW threshold current and the reciprocal external differential quantum efficiency of uncoated self-aligned lasers 2.5 jam wide at room temperature. It shows an internal differential quantum efficiency of 82% and a waveguide loss of 11.9 cm-'. Figure 27 shows the CW L - I charactersitics of AR-HR-coated self-aligned lasers of different widths at room temperature. Threshold currents of 12 and 14 mA are achieved for lasers 508 pm long with openings 2.5 and 4.5 pm wide respectively. In

1°°/RT cw

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0 /

40

~

o

~

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FAR-FIELD ANGLES (deg)

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n+-GaAs SUBSTRATE n-METAL

Fig. 25. Schematic diagram of the self-aligned index-guided InGaAs/ GaAs/InGaP quantum well laser. Patterned n-doped InGaP is used as the blocking and confinement layer with the aluminum-free regrowth surface.

0

I

0

100

,

I

200 300 C U R R E N T (mA)

r

I

400

L

500

Fig. 27. CW light-current characteristics of coated self-aligned lasers with a cavity length of 508 p.m at room temperature. The channel widths are 2.5 and 4.5 p.m. The inset shows the far-field radiation pattern of the laser 2.5 I.tm wide.

J. M. Kuo / GSMBE-grown Ino.5Gao.5 P and 11705A 1o5 P heterostructures

z

W

University of Michigan, in particular Y. J. Chan, Y. K. Chen, M. A. Chin, R. Fin, E. A. Fitzgerald, J. H. Huang, S. Hui, D. Pavlidis, S. S. Pei, A. M. Sergent and M. C. Wu.

RT, CW

G" 5O

171

25

References

Z

o 0

i

i

1000

i

r

i

1005 WAVELENGTH (nm)

I

I

r

1010

Fig. 28. Lasing spectrum of a 2.5 lam × 508 I.tm self-aligned laser at 30mW CW output power showing a single-mode emission at 1.005 lam with a side mode suppression ratio greater than 30 dB.

the absence of lateral carrier confinement in the active regions, the current spreading is high in these lasers (about 8 mA), as estimated from the dependence of threshold current on channel width from 2.5 to 12 lam. The external differential quantum efficiency is 0.68 m W m A -t at room temperature. The peak power emitted into free space is 61 m W for a laser 2.5 lam wide and 83 m W for a laser 4.5 lam wide. At 30 m W the laser 2.5 tam wide gives a 0 1 = 5 4 ° and 011= 19 ° far-field radiation pattern. The peak power is limited by the tightly confined optical modes in the cavity. Figure 28 shows the lasing spectrum of a 2.5 lam x 508 lam laser. A single-mode emission spectrum centered at 1005 nm at 30 m W output power was obtained. The side mode suppression ratio is more than 30 dB. Fundamental mode lasing was observed up to the highest power for this laser 2.5 gm wide. This demonstrates the effectiveness of the index guiding in this self-aligned structure. The lasers were able to operate in CW mode up to 145 °C.

8. Conclusions The present status of GSMBE growth and device applications of Ino.sGao.sP and Ino.sAlo.sP heterostructures has been discussed in this paper. The unique properties of Ino.sGao.sP as well as Ino.sAlo.sP and their important contributions to the field of optoelectronics have been illustrated by several examples, including MODFETs, HBTs, visible lasers and 0.98 !am lasers. More work will be required on optimizing the growth and understanding the band structures before full utilization of these materials. Acknowledgments The author gratefully acknowledges the collaboration of his colleagues at A T & T Bell Laboratories and the

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